Biogas production in rural areas of Mexico

Jevgenijs Koldisevs

Master of Science Thesis KTH School of Industrial Engineering and Management

Energy Technology EGI-2014-116MSC-EKV1074 Division of Energy Technology

SE-100 44 STOCKHOLM

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Master of Science Thesis EGI-2014-116MSC

EKV1074

Biogas production in rural areas of Mexico

Jevgenijs Koldisevs

Approved 2014-12-16

Examiner Prof. Torsten Fransson

Supervisor at KTH Miroslav Petrov

Commissioner EXERGIA, Mexico

Contact person in industry Jesus Alberto Ramos Gonzales

Abstract

Mexico is highly dependent on fossil fuels. One of the governmental initiatives proposes to investigate the possibilities for biogas production in rural areas. Agricultural producers are highly susceptible to energy price variations and in most cases the steadily growing prices of electricity and fuels are lowering the profits and income of farmers. Environmental concerns about fossil fuels give additional stimulus towards a switch to more sustainable energy sources.

Literature research on different types of biogas production technologies was carried out. Various anaerobic fermentation stages and techniques were studied in order to familiarise with particular features and to understand which of them will be more suitable for locations in rural Mexico, also taking into account the local conditions, availability of raw material, energy demands, etc.

Mexico's agricultural policies and governmental support schemes were also studied. The agricultural sector in Mexico is strategically important and brings around 6% of the country's GDP. Security and availability of energy for the rural areas is crucial. A lot of improvements need to be done in this field to make this sector attractive for investors and more profitable for people who already work there. Such governmental organizations as SAGARPA - The Secretariat of Agriculture, Livestock, Rural Development, Fishing, and Food - and its support schemes and programs are aimed to help farmers both financially and technically to overcome the difficulties concerning this procedure.

A detailed description of a proposed biogas production plant for a case study project at a milk farm in southern Mexico is presented in this study. Different stages and control strategies of biogas production as well as possibilities for biogas utilization in a small-scale CHP unit are assessed. Economical study was aimed to determine the financial flows of the sample project. A simplified economic analysis showed that the electricity produced in the biogas-driven CHP unit can compete with actual grid electricity prices. In addition, comparatively short payback time could be expected and the available governmental support schemes could be efficiency exploited for a commercially viable biogas production.

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Table of Contents Abstract ......................................................................................................................................................... 2

Table of Contents .......................................................................................................................................... 3

Acknowledgements ....................................................................................................................................... 5

List of Figures ................................................................................................................................................ 6

List of Tables .................................................................................................................................................. 6

Abbreviations ................................................................................................................................................ 7

1. Introduction............................................................................................................................................... 8

1.1 Background.......................................................................................................................................... 9

1.2 Aim and objectives ............................................................................................................................ 10

1.3 Methodology ..................................................................................................................................... 11

2. Literature review ..................................................................................................................................... 12

2.1 Introduction to biogas production .................................................................................................... 12

2.2 Biogas production by anaerobic fermentation ................................................................................. 12

2.2.1 Substrates used in anaerobic fermentation ............................................................................... 13

2.2.2 Substrates pre-treatment........................................................................................................... 13

2.2.3 Steps of Anaerobic Digestion ..................................................................................................... 19

2.2.4 Parameters of the anaerobic fermentation process .................................................................. 21

3. Evaluation of the biogas production potential in the rural areas of Mexico .......................................... 30

3.1 Overview of the energy sector .......................................................................................................... 30

3.2 Overview of the agricultural sector in Mexico .................................................................................. 31

3.3 Market supporting elements............................................................................................................. 33

3.3.1 SAGARPA .................................................................................................................................... 33

3.3.2 Agricultural finance .................................................................................................................... 34

3.3.3 Renewable energy support schemes ......................................................................................... 34

3.4 Willingness to change ....................................................................................................................... 35

4. Case study – Biogas production on the LA MONTAÑA dairy farm. ......................................................... 37

4.1 Introduction....................................................................................................................................... 37

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4.2 Biogas production unit ...................................................................................................................... 38

4.2.1 Pre-separation of slurry .............................................................................................................. 39

4.2.2 Biomass pre-treatment .............................................................................................................. 39

4.2.3 Biogas digester ........................................................................................................................... 39

4.2.4 Biogas pre-processing ................................................................................................................ 40

4.2.5 Automatics ................................................................................................................................. 42

4.3 Biogas utilization with CHP ................................................................................................................ 43

4.4 Project economics ............................................................................................................................. 44

4.4.1 Payback time analysis ................................................................................................................. 45

4.4.2 Cost of energy method ............................................................................................................... 46

5. Conclusion ............................................................................................................................................... 47

5.1 Future work ....................................................................................................................................... 48

References ................................................................................................................................................... 49

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Acknowledgements I would like to thank my wife Jolanta and other members of my family for understanding and support.

I would like to thank my industrial partner - company "Exergia" for the support and guidance during this work.

Special thanks to my mates from KTH for the good times we had there.

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List of Figures Figure 1.1 Map of Mexico Figure 1.2 Total energy consumption in Mexico, by type Figure 2.1 Biogas cycle Figure 2.2 Principles of the cavitation bubbles formation Figure 2.3 Effects of different alkali agents on COD solubilisation Figure 2.4 Stages of anaerobic digestion Figure 2.5 Relationship between relative gas output, process temperature and conservation time Figure 2.6 Growth rates for different types of microorganisms Figure 2.7 Types of digester mixing Figure 2.8 pH and ammonia relation Figure 3.1 Mexico’s oil production and consumption Figure 3.2 Mexico’s net electricity generation Figure 3.3 Agricultural map of Mexico Figure 3.4 Structure of the SAGARPA budget Figure 4.1 LA MONTAÑA dairy farm Figure 4.2 Designed biogas production plant combined with small-scale CHP Figure 4.3 Slurry separator Figure 4.4 Bioreactor Figure 4.5 Mechanical mixer Figure 4.6 Biogas psychometric chart Figure 4.7 Biogas conditioning system Figure 4.8 Biogas plant control unit Figure 4.9 CHP unit components

List of Tables Table 2.1 Characterizations of digestible raw materials Table 2.2 Relationship between process temperature and conservation time Table 2.3 Inhibitory and toxic compounds Table 3.1 Available support schemes in Mexico Table 4.1 Sample price-list for biogas production unit

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Abbreviations AD – Anaerobic digestion

CHP – Combined heat and power

C:N ratio – Carbon to nitrogen ratio

COD – Chemical oxygen demand

DM – Dry matter

GHG – Greenhouse gases

kWh – Kilowatt hour

LCCA – Life cycle cost analysis

O&M – operation and maintenance

R&D – Research and development

VFA – Volatile fatty acids

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1. Introduction The current global energy supply level is highly dependent on fossil fuels (crude oil, lignite, coal, natural gas). These resources mainly consist of carbon that was subjected to heat and pressure in the Earth’s crust over hundreds of millions of years. Therefore, fossil fuels are non-renewable resources and their reserves decrease much faster than new ones are created.

Biogas is a clean and renewable kind of energy and can enhance traditional energy sources because of its environment friendliness providing rational waste utilization and nutrient recycling [1]. Unlike fossil fuels, biogas obtained in anaerobic fermentation is a completely renewable resource because it is produced from biomass that accumulated solar energy in the photosynthesis process. Biogas is formed in the absence of oxygen, and is composed mainly of methane (50—87 %), carbon dioxide (13—50 %) and insignificant impurities of H2 and H2S. Generally speaking, locally produced biogas can improve the national energy balance as well as make a major contribution to the conservation of natural resources and environmental improvement [2].

Utilization of fossil fuels like lignite, coal, crude oil and natural gas transforms carbon that was kept millions of years in the Earth's crust into the carbon dioxide, which is released to the atmosphere. Increasing CO2 in the atmosphere causes global warming, because carbon dioxide is greenhouse gas. Biogas utilization for energy production also releases CO2; however, the main difference comparing with fossil fuels is that carbon in biogas is coming from plants that attached CO2 from the atmosphere by photosynthesis. In this way, the carbon cycle of biogas ends at a very short time (one or more years). Obtaining biogas by anaerobic fermentation also reduces methane (CH4) and nitrous oxide (N2O) emissions that result from manure storage. Methane has 23 times greater a greenhouse gas potential than carbon dioxide, while for nitrous oxide this number is 296 times greater than for CO2. As a result, utilization of biogas replaces fossil fuels in the energy mix thus reducing CO2, CH4 and N2O emissions and helping to reduce global warming [3].

One of the major advantages of biogas production is an opportunity to transform the waste into useful energy by using waste as a raw material for anaerobic fermentation. Many countries have huge challenges associated with extremely high organic waste from industrial, agricultural and household sources. Production of biogas is a great way to fulfil the increasingly strict national requirements in this area using organic waste for power generation. As a result, biogas production technologies should be seen as a help to reduce waste volume and waste disposal costs. The amount of organic waste in the rural areas of Mexico is a huge challenge but it can be seen as a domestic energy source with a potential for the country’s development and a step towards sustainability.

Development of the national biogas sector encourages the appearance of new companies. This will increase incomes in rural areas and farming communities and will create new workplaces. Compared with fossil fuels, the production of biogas by anaerobic digestion requires a much larger workforce to provide production processes, to collect and transport feedstock, to manufacture technical equipment and to install, operate and maintain biogas plants [4].

Biogas processing has no geographical restrictions and is produced primarily from raw materials that are locally accessible, which makes it an inexpensive and simple option. Another advantage is water consumption that is relatively low in comparison with other biofuels. This aspect is just as important as the production efficiency because of the future challenges for countries such as Mexico where a lack of water is expected.

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1.1 Background Mexico, officially the United States of Mexico, is a republic located in North America, which is bordered by the United States on the north and, by Guatemala and Belize on the south. The Pacific Ocean on the west and Gulf of Mexico on the east create the water borders of the country. The strategic location of Mexico, which connects together the Northern and Southern Americas, gives a lot of benefits and advantages for the successful development of the country.

Figure 1.1 Map of Mexico [5]

The total area of the country is 1 972 559 km2, which includes around 6 000 km2 of the island territory in the Pacific Ocean, Gulf of Mexico and Caribbean Sea. Population of the country is around 117 million people. Annual population increase is around 1,1%. The decrease of this number from the 3% in the middle of the 20th century is a result of governmental initiatives and the population law, which was announced in 1973. Due to the high population density in the habitable regions of the country, the state is meeting huge challenges such as waste processing, fresh water availability and energy security, especially in the rural areas [6].

Due to the reason that Mexico was a Spanish colony for 300 years, the country was a cheap source of the raw materials and the best market for the selling of goods. This type of economic model made extremely rich a very small part of the Mexican elite, which mostly consists of Spanish and Creoles, but interrupted development of the country's economy for many years. At that time, the Mexican economy was totally based on the agricultural sector - cultivation of maize, beans, chili pepper and animal breeding, and was highly dependent on the cheap local labor market. Nowadays, Mexico is a highly industrialized country, one of the leading oil and gas producers, but agricultural traditions are still very strong.

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Due to the fact that Mexico is one of the ten largest oil and gas producers in the world, the local energy sector is highly dependent on relatively cheap fossil fuels. The availability of fossil fuels is proportionally reflected in the energy mix that can be seen on the diagram below. Nowadays, consumption of oil has increased up to 56% followed by natural gas, which accounts for 29% of the total energy consumption in Mexico. Other energy sources are playing much less of a role on the market and accounted for just 15%. Non-hydro renewables (5% of the energy mix) can be mostly related to the traditional biomass consumption in the rural areas of the country.

Figure 1.2 Total energy consumption in Mexico, by type [7]

A reduction of the market share presented by fossil fuels can bring a lot of benefits for the country’s economy, such as new job creation in the sustainable energy sector, the possibility to sell produced oil on the international markets for the higher price, the reduction of the GHG emissions and creating a greener image on the international arena. Technologies such as biogas production from agricultural waste will bring a development in the agricultural sector of the country, which will allow farmers to switch from the traditional biomass towards more efficient energy production and decrease dependence from the energy prices on the international and local market.

1.2 Aim and objectives In a developing country like Mexico which is highly dependent on fossil fuels and has significant problems with energy delivery to rural areas, renewable and green energy sources should be studied and analyzed because of the possibility to increase energy security and the quality of life of the local population outside the big cities. Small-scale biogas driven CHP can be an extremely profitable and sustainable alternative. This type of energy production is designed to utilize widely available raw materials like agricultural waste in the rural areas. In addition to climate change and energy security related problems, localized biogas production from agricultural waste will be solving secondary

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problems like waste processing, will create new workplaces and definitely will help to attract new investors in the agricultural sector.

Analysis of the possibilities to change the situation in the energy sector of the Mexican rural areas towards a more sustainable and energy secure future is the main objective of this study. Study of the local particular features of the agricultural sector, energy demands of the rural households and farmers and possibilities to involve government in this biogas production project will be carried out in cooperation with the Mexican company “Exergia”. Data collected in these studies will be used for the economic analysis to prove that projects of this type might be economically attractive and profitable.

1.3 Methodology This study will be separated in 4 different phases to reach the goals described above:

• Literature review

Literature review is aimed to give a theoretical understanding of the biogas production principles.

• Information and facts gathering

Information and facts gathering will be done in cooperation with the industrial partner “Exergia”. Information from non- and governmental organizations will be collected in this phase of the project. The main objective of this stage is to understand the economic and political situation in the energy and agricultural sectors in Mexico.

• Case study and biogas production plant design

Biogas production project will be designed for the dairy farm. Technical solutions will be based on the collected data and should be seen as groundwork for the economic analysis. Detailed study of the fundamental principles and chemical aspects of the biogas production utilizing different raw materials gives a better understanding how the particular needs of the project can be met taking in account local features of the agricultural sector.

• Economical assay

The main objective of the economical assay is to show that a project of this type might be profitable for the investors and energy producers. Simplified economic analysis will be used for a better understanding of the economic aspects of the project and comparison with other energy types available on the Mexican market.

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2. Literature review

2.1 Introduction to biogas production The nutrient cycle of the biogas production process - from raw material production until the usage of recycled substrate as a fertilizer – is closed. Carbon (C) compounds are reduced in the fermentation process because methane (CH4) is used for energy production and carbon dioxide (CO2) is released into the atmosphere from where plants capture it in the photosynthesis process. Some of the carbon compounds remain in the recycled substrate and in using it as a fertilizer, soil carbon content is improved. Recycled substrate of fermented biomass is rich with nitrogen, phosphorus, potassium and microelements. Compared with untreated manure, revised substrate has improved fertilizer efficiency because it is homogeneous, richer in nutrients, has better C / N ratio and is almost odourless [8].

Figure 2.1 Biogas cycle [3]

Biogas production can be perfectly integrated into ordinary and organic farming because recycled substrate replaces mineral fertilizer that is produced using large amounts of fossil energy. The closed sustainable biogas cycle is shown in the figure 2.1.

2.2 Biogas production by anaerobic fermentation Anaerobic fermentation is a biochemical process during which a variety of organic substrates (plant biomass and waste, manure and slurry, organic waste and wastewater, wastewater sludge) break down by the action of different types of bacteria in the absence of oxygen, creating biogas and recycled substrate. Perceptible growth in the variety of the substrates used for anaerobic fermentation has been seen recently.

The technological procedure of standard anaerobic digestion is made up of three basic steps: (i) substrate preparation and pre-treatment, (ii) anaerobic digestion, (iii) post treatment of digested substance (comprising biogas use) [9].

In the anaerobic digestion process there are the 3 following pre-processing methods - physical, chemical and biological. The physical pre-treatments are based on thermal, ultrasonic and mechanical processes such as ultrasound, high-pressure homogenizer, mechanical jet and mechanical ball mill. In the chemical pre-treatments group, belong alkali and organosol processes, ozonolysis, acid hydrolysis

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and wet oxidation. Biological pre-processing methods (microorganisms) as well as combined pre-treatments (thermochemical or alkali-thermo) also can be applied [3].

2.2.1 Substrates used in anaerobic fermentation If the biomass contains carbohydrates, fats, proteins, cellulose and hemicellulose as major elements, then it can be utilized as substrate for anaerobic fermentation. Below are listed the most common types of raw materials:

• Manure and slurry;

• Agricultural remains and by-products;

• Digestible waste of food industry and agro industrial waste (plant and animal origin wastes);

•Organic fractions of municipal waste and catering waste (plant and animal origin wastes);

• Wastewater sludge;

• Special energy crops;

Usage of manure and slurry as an anaerobic digestion material has several advantages [3]:

• Natural presence of the anaerobic bacteria;

• High water content (4-8% of solid material in the slurry), which operates as a solvent for added substrates, ensures proper mixing of biomass and process flow;

•Low price and high availability;

Substrates for anaerobic fermentation can also be classified according to their origin, their dry matter (DM) content, methane outcomes and other criteria. Table 2.1 further below provides an overview of some properties of typical fermentable materials.

Substrates, with dry matter content less than 20%, are used in the so-called wet fermentation. If the dry matter content is at least 35%, it is called dry fermentation and is commonly used in the case of energy crops and silages [10]. The choice of type and quantity of raw material for anaerobic fermentation depends on dry matter, carbohydrate, protein and lipid content; in addition the C/N ratio in fermentation substrates should be between 25 and 35 [11].

2.2.2 Substrates pre-treatment Altering physical or chemical properties of the substrates in the pre-treatment process can increase the anaerobic digestion processes and make biogas production process more beneficial. Pre-treatment methods of substrates have several advantages: an increase in the economical profitability as well as reduction of the nitrogen limits that shortens degradation of activated substrates [12]. The aim of the pre-treatments phase is to transform the structure of complex elements (generally cellulosic) with reducing order of polymerization, the weakening of the chemical bonds of lignin [13]. Below different classes of physical (mechanical, thermal and ultrasonic), chemical and biological pre-treatment methods are discussed and analysed.

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Table 2.1 Characterizations of digestible raw materials

Type of raw material

Organic substance

C:N

Ratio

Dry matter %

Biogas outcome m3/kg volatile solid

Adverse

physical impurities

Other undesireble substances

Pig slurry Carbohydrates,

proteins, lipids

3-10 3-8 0.25-0.50 Wood chips, bristles, sand, water, cords,

straw

Antibiotics,

Disinfectants

Beef slurry

Carbohydrates,

proteins, lipids

6-20 5-12 0.20-0.30 Bristle, wood, soil, water, straw

NH4+,

antibiotics,

Disinfectants

Poultry slurry

Carbohydrates,

proteins, lipids

3-10 10-30 0.35-0.60 Grift, feathers, sand

NH4+,

antibiotics,

Disinfectants

Whey 75-80% lactose

20-25% protein

8-12 0.35-0.85 Carriage impurities

Flotation sludge

65-70% proteins

30-35%lipids

Animal tissues

Organic pollutants,

heavy metals,

disinfectants

Straw Carbohydrates,

Lipids

80-100 70-90 0.15-0.35 Sand, grit

Garden wastes

100-150

60-70 0.20-0.50 Soil, cellulosic

components

Pesticides

Grass

12-25

20-25

0.55

Grit

Pesticides

Fruit wastes

35 15-20 0.25-0.50

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2.2.2.1 Physical pre-treatment Physical pre-treatment is basically the process of the destruction of biomass particles by milling or shredding. An increased rate of hydrolysis and the anaerobic biodegradability of substrates are reached in this process. Hydrolysis development arises through the crystallinity decrease as a result of the reduction of the particle size. Required particles size determines the energy requirements for physical pre-processing. In many physical pre-treatment cases, the required energy amount for biomass pre-processing might exceed the theoretical energy approachable in the biomass [14].

2.2.2.2 Mechanical pre-treatment Mechanical pre-treatment process results in a reduction in the biomass particles size and refines accessibility for microorganisms by breaking cell walls of the substrate and making components more biodegradable by improvement of the rate and effectiveness of hydrolysis [12]. Mechanical jet, high-pressure homogenizer, mechanical ball mill are the most known mechanical pre-treatment techniques that are mostly used for municipal waste pre-treatment. The basic procedures that are applied in the process of division and handling of substrates are listed below:

1. Separation by size process implicates the separation of a composition in two or more fractions by using different screening surfaces. Vibrating screens, trammel screens and screens on disk are widely used forms of sieves in the division of municipal solid waste.

2. Separation by density process is based on division of the substances to the light fraction (paper, plastic and organic) and dense fraction (metals, wood, etc.). Applied techniques are pneumatic classification, flotation and dense media separation.

3. Separation by electromagnetic field process is applied for the restoration and division of ferrous and non-ferrous metals. For this process, special technologies are used - magnetic separation, eddy current separation and electrostatic separation.

4. Compaction process is applied for increasing the productivity of storage and carriage of substances. In this process different granulation and packaging techniques are applied.

Although biogas production is amplified by various mechanical pre-processing methods, the energy consumption is very high.

2.2.2.3 Thermal pre-treatment Thermal pre-treatment is characterized as a method that refines the productivity of anaerobic digestion process. The reason is that thermal hydrolysis leads to an increase of solubility of organic substances and inorganic compounds and as a result enhances biogas production [15]. Thermal pre-treatment application primarily was concerned with refining sludge dewater ability and improving solid destruction. Conventionally, carbohydrates and lipids within the sludge are easy to destroy compared to proteins where the cell wall preserves them from the enzymatic hydrolysis. The thermal pre-treatment techniques are aimed to break down cell walls and promote proteins to be approachable for biodegradation [16].

One of the studies shows that some thermophilic bacteria populations are biologically activated by thermal pre-treatment at temperatures around 70 ◦C. In this case, low temperature pre-treatment can be seen as a pre-digestion stage [17].

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Anaerobic digesters have two common exploitative temperature levels, which establish the presence of mesophilic or thermophilic microorganisms in the digesters. The mesophilic microorganisms mainly exist at temperatures around 20-35°C and thermophilic microorganisms live optimally around 45-55°C. Thermophilic bacteria can withstand elevation of the temperatures up to 70°C where they are the basic existing microorganisms [12].

Some studies are attempting to classify biomass thermal pre-treatment technologies in conformity with the effect on biogas manufacture:

1) Biomass pre-treatment at temperatures between 70°C and 121°C results in a 20-30% biogas output growth

2) Biomass pre-treatment at temperatures between 160 and 180°C leads to significant biogas yield rise up to 40-100% [18].

However, pre-treatment temperatures above 200°C act as an inhibitor for the digestion process and reduce biodegradability because of the rise of nitrogen and phosphorus concentration in the substrate due to hydrolysis of microbial cell constituents [12].

It is important to mention that the nature of the substrate determines temperature conditions and continuance of pre-treatment, however, high-temperature conditions for the thermal pre-treatment process requires high energy consumption.

2.2.2.4 Ultrasonic Pre-treatment In the ultrasonic pre-treatment method, vibration waves are used as a basic force of degradation. Ultrasound-assisted pre-treatment is an effective and founded mechanical pre-treatment method to increase biodegradability of the biomass. It increases the biomass digestibility by changing the biological, physical and chemical features [19].

Ultrasonic sound is a sound with the frequency above the human audible diapason (higher than 20 kHz). However, disagreeable lower frequencies can be audible during the operation. Ultrasound is translated through any material medium. As a result, waves squeeze and expand the molecular sites through which they penetrate. Ultrasound crossing the medium changes the mean distances between the molecules as they vibrate around their average position, and as consequence voids are created [20].

Cavitation is the decisive degradation mechanism that decreases the size of particles in the ultrasonic pre-treatment method. Variable high pressure and low-pressure series are applied when a permanent ultrasound wave is spread into a liquid at high strength. Low-pressure periods allow the dissolved gases to achieve the vapour pressure and, thereby, build fine vacuum bubbles. After a while, the bubbles achieve a capacity at which they can no longer hold the equilibrium between the viscosity and pressure intensity, and the violent explosion of bubbles occurs during the high-pressure cycle [21]. However, a specific power should be achieved to provide the crushing of the bubbles. In fact, it is accepted that two different types of cavitation exist: stable or transient. At the low ultrasonic intensities of the waves (1- 3 W/cm2) sustained bubbles are built and they vibrate around some equilibrium dimension for many acoustic cycles. In the case of transient cavitation, bubbles are generated applying ultrasound intensities larger than 10 W/cm2. Bubbles developed through a transient cavitation have a radius of at least twice their original volume, before breaking on the compression [20]. The figure below shows the principles of the cavitation bubbles formation, where R is a rarefied region and C is a compressed region; (a) Transfer (x) graph; (b) Depiction of transient cavitation;(c) Depiction of stable cavitation; (d) pressure (P) graph.

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Figure 2.2 Principles of the cavitation bubbles formation [20]

An important parameter of this process is the Ultrasonic intensity (I) which represents the power, passing trough the probe cross-section area per unit time and can be calculated using the following equation [22] (U Neis, 2000):

𝐼 =𝑃 ∗ 𝑡𝐴

Where: I - ultrasonic intensity is in W/cm2;

P = ultrasonic power in W;

t = ultrasonication time in seconds;

A = surface area in cm2;

From the equation it is seen that ultrasonic intensity will grow proportionally with the generator power amplitude. The threshold intensity magnitude for cavitation creation is 0.1 W/cm2, which is considered as a low frequency magnitude. Destruction of cellular substrates is most considerable at low frequencies, because the bubble radius size is inversely proportional to the frequency and large bubbles denote powerful shear forces [23]. However, to cause significant degradation, 10 W/cm2 is necessary for transient cavitation bubbles [20].

2.2.2.5 Chemical Pre-treatment Chemical pre-treatment methods can be divided into alkali and acid hydrolysis pre-treatment. Such technologies as organosolv process, wet oxidation (processing involving water and oxygen at temperatures above 120°C) and ozonolysis pre-treatment can also be included into the chemical pre-treatment group.

The alkaline method is the most widely used chemical pre-treatment method, which is able to increase the efficiency of solubilisation of COD (chemical oxygen demand) and biogas extraction. Organic particles swell into the alkaline medium, making them more sensible to enzymes by enhancing the biodegradability in the solid state. Alkaline pre-treatment of substrates in anaerobic digestion is performed by adding alkaline solution on the substance at 25°C for 24 hours reaction time. Eventually, the sample is filtered and a liquid and solid fraction is received [12].

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Various studies show different effects of alkaline solutions such as NaOH, KOH, Mg(OH)2 and Ca(OH)2. Some research shows that the COD solubilisation value after addition of NaOH, KOH, Mg(OH) and Ca(OH) at room temperature was 39.8%, 36.6%, 10.8% and 15.3% respectively [24].

Figure 2.3 Effects of different alkali agents on COD solubilisation [24].

Generally, Figure 2.3 specifies that the solubilisation of monobasic agents is higher than for dibasic alkalines that are just partly dissolved.

Organic wastes can be solubilized by alkaline pre-treatment and biogas output enlarges, but the biogas output cannot grow linearly with the concentration of the NaOH solution [25]. Different studies show that optimum alkaline pre-treatment conditions with the addition of NaOH should be pH=12 for an increase in the biogas output [26]. (Alexandre Valo, 2004) Other studies showed that the optimal temperature for applying alkaline pre-treatment is around 130°C to 140°C, but higher temperatures can frequently enhance COD solubilisation, however biogas output doesn’t increase [27].

Some research shows that ammonia can also be utilized as a pre-treatment agent with the following advantages: improved nitrogen supply for biodegradation, increased biogas production and decreased pre-treatment time. This method is commonly used for large-scale biogasification of corn straw. Another parameter of the process is carbon to nitrogen correlation (C/N), which for an effective anaerobic digestion process can vary from 25% to 35%. For more effective digestion, nitrogen can be added in the form of ammonia, urea, and animal manure or food wastes in certain organic substrates for an increase in the nitrogen proportion. Another advantage of ammonia usage is high commercial potential because of regeneration and recycling possibilities due to NH4+’s great volatility. As a result, chemical price and residues processing decrease [12].

2.2.2.6 Biological pre-treatment The main objective of biological pre-treatment is biomass cell degradation by enzymes obtained from microorganisms. In nature, a few bacteria types and some fungi can be used to degrade organic substrates [12].

0

5

10

15

20

25

30

35

40

45

NaOH KOH Mg(OH)2 Ca(OH)2

COD

sol

ubili

sati

on (%

)

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Different types of substrates (sludge, household waste, industrial waste, slurry) require usage of various enzymatic applications - special species of fungi or combinations of them (Aspergillus oryzae, Aspergillus niger etc.) [12]. Fungi are able to release enzymes that can break down lignin, hemicellulose, and polyphenols. White- and soft-rot fungi are effective in breaking down the lignocellulose substance, while white-rotare is very useful for a biological pretreatment of biomass [28]. Sphingomonas paucimobilis and Bacillus circulans bacteria were used as pretreatment method in some studies of office paper production and results showed improvement of the enzymatic hydrolysis. Office paper displayed a sugar reconstruction increase up to 94% [12]. Some studies evaluated biological pretreatment of orange peels by applying fungal strains of Penicillum, Aspergillus and Fusarium and the pre-treatment demonstrated an increased presence of feed constituents and decreased level of the antimicrobial content [29].

The major advantages of biological pre-treatment are moderate environmental impact, low energy consumption and no chemical demand. However, the rate of biological pre-treatment is very slow in most cases, especially for industrial applications. Disadvantages that should be mentioned are long employment period of 10–14 days, the demand of accurate growth provision as well as significant amount of area need for the biological pre-treatments applications [30].

2.2.3 Steps of Anaerobic Digestion Anaerobic digestion is a collection of controlled decomposition processes under adjustable conditions in the absence of oxygen. Temperatures suitable for the anaerobic digestion process are in the range of naturally living mesophilic or thermophilic anaerobic and facultative bacteria conditions. The optimal temperature region for mesophilic anaerobic bacteria is 35-40°C, while thermophilic bacteria have maximal potential at range of 50-55°C. Functions of various anaerobic bacteria are different in their assignments at different stages of the digestion. The process of anaerobic digestion can be split into four steps: hydrolysis, fermentation, acetogenesis and methanogenesis. Figure 2.4 gives a better interpretation of stage division of anaerobic digestion [3].

Figure 2.4 Stages of anaerobic digestion

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2.2.3.1 Hydrolysis In the first stage of anaerobic digestion, an extracellular process occurs. This process includes reconstruction of the complex organic matter (protein, carbohydrates, fats and cellulose) to soluble oligomers and monomers by means of enzymes. Hydrolysis of the organic substance can be performed by several methods: adding enzymes or applying chemical or thermal pretreatment. The hydrolysis itself includes individual steps such as enzyme extraction, diffusion, adsorption, reaction, and enzyme decontamination [31]. The output of enzymatic hydrolysis contains monosaccharide, fatty acids and amino acids. Cellulase, xylanase and amylase are hydrolytic enzymes used for destroying carbohydrates into sugars. Protease is applied into the process to obtain amino acids from protein, but lipase is used for breaking down lipid into fatty acids [32].

Some studies showed that in case of substrates with large particle sizes, for example pig waste and cattle manure, hydrolysis is a rate-limiting stage for digestion, while in the case of easy degradable substances, methanogenesis is the rate-limiting stage [33].

2.2.3.2 Acidogenesis In the acidogenesis stage, fermentative microorganisms compose decomposition of soluble organics to carboxylic groups. Hydrolysis products penetrate into bacterial cells where further converting takes place. Acidogenesis is partly performed by anaerobic bacteria that absorb the residue of oxygen consequently assuring appropriate anaerobic atmosphere for the methane bacteria. The original substance and environmental conditions dictate the ultimate products of the metabolic activities of bacteria. For instance, glucose metabolism reactions can occur in different way:

C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2 (Glucose → Acetate) 3C6H12O6 → 4CH3CH2COOH + 2CH3COOH + 2CO2 + 2H2O (Glucose → Propionate + Acetate) C6H12O6 → CH3CH2CH2COOH + 2CO2 + 2H2 (Glucose → Butyrate) C6H12O6 → 2CH3CHOHCOOH (Glucose → Lactate) C6H12O6 → 2 CH3CH2OH + 2CO2 (Glucose → Ethanol)

The first reaction has a larger preference in the biogas production process because of production of acetic acid, which is the main precursor of CH4. The reaction with butyrate formation occurs when there is an accumulation of hydrogen in the structure [34]. Some factors such as concentration and pH determine the prevailing route of the reaction. If there is very high organic content, lactic acid output will be considerable. Low pH (< 5) raises the output of ethanol [35].

2.2.3.3 Acetogenesis In this stage, products of acidogenesis formation, which can’t be directly converted to methane, are transformed to the methanogenic substrates in the presence of methane forming bacteria. Volatile fatty acids (VFA) and alcohols are oxidized into methanogenic substrates: acetate, hydrogen and carbon dioxide. Volatile fatty acids, with carbon chains that are longer than two atoms, and alcohols, with carbon chains longer than one atom, are oxidized into acetate and hydrogen. The manufacture of H2 enlarges the hydrogen partial pressure. Low hydrogen partial pressure is relevant for acetogenic reactions to be thermodynamically beneficial (ΔG>0). H2 is regarded as waste product of acetone group formation stage and prevents the metabolism of the acetogenic group bacteria [3].

In the acetogenesis stage, propionate is mostly oxidized via the methyl-malonyl-CoA route, producing acetate, hydrogen and carbon dioxide:

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CH3CH2COOH + 2H2O → CH3COOH + 3H2 + CO2; ΔG0 = 76.2 kJ mol;

Butyrate also decomposes into acetate:

CH3CH2CH2COOH + 2H2O → 2CH3COOH + 2H2; ΔG0 = 48.4 kJ mol;

As it is seen from reactions, acetate is the essential intermediate in the transformation of organic substances into methane and carbon dioxide. The formation of acetone groups and the methane formation phase generally occurs in parallel as a symbiosis of two kinds of microrganisms. Acetate gives around 70 % of the total methane generated in anaerobic digestion [36]. The acetogenic bacteria grow very slowly and are susceptible to changes in the organic substance as well as environmental modifications, and therefore a long time is demanded for bacteria to adapt to new environmental conditions [37].

2.2.3.4 Methanogenesis Methanogenic bacteria accomplish a methane and carbon dioxide extraction from intermediate products. Methanogenic bacteria are able to cultivate on a limited number of substrates such as H2/CO2, acetate, formate, alcohols and methyl group compounds (methanol, methylamine) [80].

Methanosaeta kind of methanogens can only dispose acetate as a substrate; however Methanosarcina kind of methanogens can utilize acetate, methanol and H2/CO2 [38].

70% of produced methane is coming from acetate, while the remaining 30% originates from hydrogen and CO2 transformation using the following reactions:

CH3COOH → CH4 + CO2 4 H2 + CO2 → CH4 + 2 H2O

Formation of methane is a decisive stage of the fermentation process as it is the slowest step of the biochemical reactions. Working conditions significantly affect the formation of methane. Composition of feed, hydraulic loading rate, temperature and pH are examples of factors affecting the methanogenesis stage [3]. The process becomes less settled if hydrogen partial pressure grows and such changes can drive the accumulation of volatile fatty acids and lead to the reduction of pH. In this case, the methanogensis stage becomes uncontrollable and the whole anaerobic digestion process fails. That is why hydrogen is acknowledged as an indicational monitoring parameter in the general anaerobic biomass digestion process [39].

2.2.4 Parameters of the anaerobic fermentation process Anaerobic digestion efficiency depends on some key parameters, so it is very important that anaerobic microorganisms are provided with suitable conditions. Their growth and activity is significantly affected by the lack of oxygen, temperature, pH and buffering systems, nutrient supply, stirring intensity, solubility of gases as well as presence of inhibitors. Methane bacteria are very sensitive to the anaerobic conditions, so oxygen during digestion process must be certainly removed [40].

2.2.4.1 Temperature The choice of temperature scale and control is crucial for anaerobic fermentation. Temperature required for the process is provided by floor and wall heating inside the bioreactor. In practice, the temperature of the production depends on the raw materials used.

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The process of anaerobic fermentation can be separeted into three temperature groups: psychrophilic (below 25oC), mesophilic (25oC – 45oC), and thermophilic (45oC –70oC). As table below demonstrates, there is a direct relationship between process temperature and conservation time of the substrate.

Table 2.2 Relationship between process temperature and conservation time [3]

Thermal period Process temperatures Minimum conservation time

Psychrophilic < 20 °C 70 to 80 days

Mesophilic 30 to 42 °C 30 to 40 days

Thermophilic 43 to 55 °C 15 to 20 days

Relative biogas output is a parameter depending on temperature and substrate conservation time. The relation between biogas output, process temperature and conservation time are illustrated in figure 2.5.

Figure 2.5 Relationship between relative gas output, process temperature and conservation time [3]

Due to differences in the process parameters, and depending on the temperature of anaerobic fermentation, the technology should be chosen depending on the other parameters of the biogas production process – geographical location of the production, climate and energy availability. For example, mesophilic bacteria are stronger in surviving in different environmental parameters. Mesophilic temperature region is appropriate for badly insulated digesters or digesters established in cold temperatures. On the other hand, operating systems of biogas production in European countries works mostly at the temperatures in the thermophilic region, because of higher growth rates of methane forming bacteria. Thermophilic anaerobic digestion process ensures a lot of advantages compared to mesophilic and psychrophilic process:

Efficient collapse of pathogens; Shortened conservation time, making the process more rapid and efficient; Advanced digestibility and accessibility of substrates; Improved degradation of solid substance and better usage of the substrate; Better separation options of liquid and solid fractions; Faster growth of methanogenic microorganisms at increased temperatures;

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However, the main disadvantages of thermophilic process are:

Greater degree of imbalance; Requires more energy for heating due to higher temperatures; Sensitive to toxic substances (ammonia inhibition);

Another parameter dependent on the process temperature is the toxicity of ammonia. Growth of temperature leads to ammonia toxicity increase, and it can be removed by reducing the process temperature. However, reducing the temperature to 50°C or below will decrease the growth speed of thermophilic bacteria and there can appear a leaching risk of microbial populations because growth speeds are lower than the real conservation time of the substrate. In this case a well-functioning thermophilic bioreactor is a good alternative because it can be more loaded and operated at lower conservation time of the substrate than mesophilic due to the thermophilic organism’s growth rates which are higher than for the mesophilic species (figure 2.6). Some temperature ranges are less favorable for operation. For instance, mesophilic bacteria can grow up to 47°C, and thermophilic bacteria can already be present at 45°C. In this case, this lowers the rate of reaction, competing in the overlapping region and preventing growth of each other. This leads to bad productivity of the process, and therefore, these temperatures are seldom employed [9].

Figure 2.6 Growth rates for different types of microorganisms [3]

Experience has shown that operation conditions with high load or low conservation time give higher gas yield and a larger transformation index for bioreactors operated at thermophilic rather than at mezophilic mode. Viscosity of fermentation compounds is inversely proportional to temperature. Substrate is more liquid-like at higher temperatures, and in this way diffusion of dissolved material is promoted. Thermophilic operation temperature provides faster chemical reactions and thus ensures larger efficiency methane production, increased solubility and lower viscosity. A high requirement of energy for the thermophlic process is justified by the larger biogas output.

It is important to mention that a constant process temperature is a crucial condition in the digestion process because fluctuations of the process temperature can negatively influence the biogas production. Thermophilic microorganisms are much more sensitive to temperature changes in the range of + / -1 °C and they need more time to adjust to the new temperature region and achieve the maximum point of methane production. Mesophilic microorganisms are less sensitive and they endure temperature changes of +/- 3°C without considerable diminution in methane production [3].

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2.2.4.2 Optimal pH values The anaerobic digestion process is greatly dependent on the pH values because each of the bacterial class included in the process has a certain optimal pH for growth. Such parameters as a carbon application, productivity of substance dissimilation, synthesis of different types of storage matters, and the liberation of metabolic outputs from the cell are affected by pH region of the process [41]. The value of pH affects the growth of methane-forming bacteria and may influence anaerobic dissociation of some important compounds (ammonia, sulfides, organic acids). Formation of methane takes place within a comparative narrow pH range, from about 6.5 to 8.2, with the optimum range between 7 and 8 for the majority of methane-forming bacteria, while the acid-forming bacteria in many cases has a lower optimum value of pH [42].

With increasing temperature CO2 solubility decreases. Therefore, the pH value of the thermophilic bioreactors is higher than in mesophilic ones because dissolved carbon dioxide reacting with water creates carbonic acid. Ammonia resulting from degradation of proteins or obtained in a feed stream can increase pH value, but accumulation of volatile fatty acids reduces the pH [43]. However, cow and pig manure digesters generally have high level of bicarbonate buffering capacity and a high amount of ammonia, which results in pH stability around 7.5-8.0 and the system can withstand quite high concentrations of volatile fatty acids before pH falls [44].

Another significant indicator in anaerobic digestion systems is alkalinity, which evaluates chemical buffering capacity of the aqueous phase. pH value in anaerobic reactors is mostly monitored by the bicarbonate buffer system, so pH value in the biogas digester is influenced by the partial pressure of CO2 and concentration of alkali and acid elements in the liquid phase. If accumulation of acidic or basic components happens, buffer capacity neutralizes the shifts in pH up to a certain level. When the buffer capacity is overrun, the pH values show significant changes completely inhibiting the process [45]. The main buffer in anaerobic digesters is bicarbonate, while hydrogen sulphide, dihydrogen phosphate and ammonia are also considerable compounds for ensuring neutralization in the system [46]. The mixture of the substrates and the total organic amount determine an appearance and concentration of buffering elements. If the pH decrease is observable in an anaerobic bioreactor, it is recommended that the reactor substrate supply is stopped for the time so that methanogenic archaea can rework the acids and the buffering capacity of the substrate is increased. However, addition of the neutralizing elements such as Ca(OH)2, Na2CO3 and NaHCO3 can normalize and adjust pH [9].

2.2.4.3 Volatile fatty acids (VFA) Volatile fatty acids are intermediate compounds with carbon chains six or less atoms long that are generated during the acid formation process (for example, acetate, propionate, butyrate and lactate). Stability of the anaerobic fermentation process and concentration of intermediate compounds like volatile fatty acids are linked to each other, that is why this product’s concentrations should be monitored [47].

Monitoring of pH, partial alkalinity and VFA is very useful for operation controlling in a weak buffered system, however in strongly buffered system merely VFA monitoring is secure for pointing out process imbalance [33]. Process instability can lead to accumulation of VFA in the bioreactor that can decrease pH in the system. However, the accumulation of VFA does not always lead to a pH drop due to buffer capacity of different types of biomass in the digester. For example, manure has an additional alkalinity, which means that the VFA accumulation must overrun a certain level before it becomes noticeable due

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to a considerable drop of pH value. In this case, the VFA concentration in bioreactor could be so high that the anaerobic digestion process will be significantly delayed [3].

On the other hand, buffer capacity of the same anaerobic fermentation substrate may vary. The Danish experience shows that buffer capacity of cattle manure varies over the seasons, which is likely affected by the composition of cattle feed. Therefore, pH value of domestic animal manure is variable, and it is difficult to use it for detecting instability of the process. However, it is important to note that the pH value can a be fast, reliable and relatively inexpensive way to determine possible instability in a weak buffer system, such as various types of wastewater [3].

However, some studies have shown that a drop in pH value accompanied with accumulation of VFA is the major reason of toxicity and bioreactor collapse in anaerobic digestion [48]. Therefore, for continuous digestion processes it is important to keep VFA at a low quantity in the reactor. The optimum amount of VFA, expressed as acetic acid, should be as low as 200 mg/L [49].

2.2.4.4 Mixing intensity Many substances and different natures of microorganisms involved in the fermentation process demand mixing application in order to preserve stability within the bioreactor. Different studies mention following pros for applying mixing to the anaerobic fermentation [50]:

1. Mixing ensures an intimate contact between the bacterial population in the digester and incoming fresh substrate

2. Helps to take out the metabolites generated by the methanogenic microorganisms 3. Mixing precludes build up of a surface crust/scum in a reactor 4. Helps to avoid thermal stratification within the digester 5. Prevents sedimentation of sand and heavy solid particles 6. Helps to reduce particle size

Mixing of bioreactor substances can be guided in different ways. For example, a good mixing result offers daily nourishment of slurry instead of recurrent feed [51]. Stirring can also be accomplished by applying certain mixing devices or techniques such as mechanical mixers, recirculation of slurry, or injection of the produced biogas [52].

1. Flat blade turbines with slow stirring speed are most suited for mechanical mixing and are usually applied for digesters with fixed covers. The sludge is conveyed by the rotating impeller(s), thereby blending the content of the bioreactor.

2. Centrifugal pumps, commonly inducted in an internal or external shaft tube to maintain vertical blending, ensures slurry recirculation. In applying a centrifugal pump, firstly, a large amount of the sludge is withdrawn from the center of digester. Secondly, sludge is pumped through external heat exchangers where it is mixed with the raw sludge and heated to a certain temperature. Feedstock is then pumped back into the bioreactor through nozzles at the ground level or at the top of the digester to break the scum. The limitation of this process is that the stream velocity in the recirculation should be large enough to provide a complete blending. The minimum power demanded is 0.005–0.008 kW/m3 of digester capacity and may be even higher. Other shortages of this technique are plugging of the pumps by solids, impeller wear from grit and bearing failures.

3. One of the most perspective stirring technologies is the biogas recirculation technique where the formation of scum is avoided. In unconfined gas mixing systems, the gas is gathered at the top of the

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digester, compressed and then set free through a pattern of diffusers or a row of radially located lances hung up from the top of the digester [52].

Figure 2.7 Types of digester mixing [52]

The picture above illustrates different types of digester mixing technologies: (a) External, pumped recirculation; (b) Internal, mechanical mixing; (c) External, gas recirculation [52].

The feedstock in bioreactor is also blended naturally by gas bubbles released during the AD process which rise up and move the digested substance to the surface. That is why scum has to be monitored in particular. Formation of scum is successfully hindered by the lance system, but it does not prevent precipitation of solids in the digester bottom. The diffuser equipment established at the bottom of the digester is efficient against solid precipitation but is not sufficient enough to control scum formation [45].

Different studies on the influence of blending forces (minimal, slow or intense) during biogas production have demonstrated that minimal and slow mixing with a low ratio of substrate to inoculum or slow blending with high substrate to inoculum ratio would generate larger methane output. However, intense blending would cause prevention of production and lower output, particularly under high substrate to inoculum proportion [53].

2.2.4.5 Inhibitory compounds The presence of a multiplicity of inhibitory and toxic compounds in varying concentrations is the main cause of system instability, disturbance of biogas production and organic removal or even digester failure. Inhibitory substances can appear in an anaerobic digestion system with the raw materials or can arise during the process [43].

Microorganisms use some substances in small concentrations for growth; however, these substances can inhibit the digestion at higher concentrations. For example, high concentrations of total volatile fatty acids can have a similar impact [43]. The majority of general inhibitors are generated during degradation of the substrate, such as ammonia, volatile fatty acids, long-chain fatty acids, and sulfide. However, some inhibitory compounds exist already in the substrate, such as heavy metals, long-chain fatty acids and antibiotics [54]. Such heavy metals as chromium, lead, zinc, cadmium, copper and nickel can induce contraventions in the anaerobic digestion process (Table 2.3). Some studies showed that copper and zinc were the most toxic heavy metals, while lead displayed weakest toxicity to acidogens [55]. Other studies have informed that copper and zinc manifest inhibitory action in batch digesters in

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the range of 1-10 mg/dm3 for copper and 5-40 mg/dm3 for zinc, indicating that copper has a higher toxicity than zinc to acidogens [56]. It is important to mention that zinc is notably present in pig slurry, arising from pig feed which contains a zinc additive as an antibiotic.

Table 2.3 Inhibitory and toxic compounds

Metal Inhibition start1 [mg/L] Toxicity to adopted microorganisms2 [mg/L]

Cr3+ 130 260

Cr6+ 110 420

Cu 40 170

Ni 10 30

Cd 70 600

Pb 340 340

Zn 400 600

1Toxic concentration that corresponds to the reduction of the biogas production

2Toxic concentration that corresponds to biogas production diminishing by 70 %;

However, heavy metals do not generally create crucial problems in a bioreactor, since sulfide and carbonate precipitation retains the ionic concentration at a low level. A bioreactor with high solids quantities can endure larger amounts of heavy metals [57].

Other important inhibitory compounds are the inorganic salts such as sodium, potassium, calcium, and magnesium, which are generally present in the digestate from bioreactors. Light metal ions may be generated by the degradation of organic substances in the feedstock or by chemical addition for pH alignment. Reasonable concentrations of light metal ions are necessary to stimulate microbial growth, however inordinate quantities will decelerate growth, and higher concentrations can lead to intense inhibition or toxicity. Salt toxicity is connected with the dehydration of bacterial cells due to osmotic pressure [43]. Some studies compared toxicity of inorganic salts on a molar concentration basis and found that monovalent cations, such as sodium and potassium, were less toxic than the divalent ions, such as calcium and magnesium [58].

Ammonia is also a degradation compound created during anaerobic digestion of solid refuse by the process of degradation of nitrogenous substances in the form of proteins, phospholipids and nitrogenous lipids [59]. Ammonia is an important nutrient and has a considerable function for the anaerobic fermentation. However, too high concentrations of ammonia, particularly in the free form, are responsible for process delay. Therefore, ammonia concentration should be retained below 80 mg/l [3]. Laboratory research has proven that the inhibition of ammonia is due to the variation of intracellular pH, the growth of the retention energy requirement to overpass the toxic conditions, and inhibition of particular enzyme reactions [60]. Inhibition of anaerobic digestion by ammonia (NH3/NH4

+) is a well-studied process. It is characteristic for anaerobic digestion of manure, as it contains a relatively high NH4

+/NH3 concentration resulting from urine. Methanogenic bacteria are particularly sensitive to ammonia inhibition. Free ammonia concentration is directly proportional to temperature, so there is an

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enlarged risk of ammonia induced inhibition for anaerobic fermentation that takes place in themophilic temperatures compared to mesophilic ones. Free ammonia, NH3, is a fraction that directly causes inhibition. Concentration of free ammonia can be calculated by an equilibrium relationship:

[NH3] =[T- NH3]/(1 + H+/Ka)

Where: [NH3] is a free ammonia concentration;

[T-NH3] is a total ammonia concentration;

Ka is the dissociation parameter (values rising with temperature);

The figure below (Figure 2.8) shows that increases in pH and temperature will also enlarge inhibition as these factors will increase the fraction of free ammonia. When the process is hindered due to ammonia, an increase of the concentration of volatile fatty acids will decrease pH. This will partly neutralize the effect of ammonia due to a decrease of the concentration of free ammonia [3].

Figure 2.8 pH and ammonia relation [61]

It is important to mention the influence of organic substances to the anaerobic digestion process. Generally, the effect on the fermentation process is dependent on organic substance concentration, biomass concentration, toxicant exposure time, cell age, feeding pattern, acclimation, and temperature [62]. Hydrophobic organic pollutants accumulate in bacterial membranes and induce swelling and leaking of the membrane, destroying ion gradients and finally creating the breakdown of cellular membranes [43]. Significant inhibitory organic substances are: chlorophenols, halogenated aliphatic, nitrogen substituted aromatic, long-chain fatty acids and lignin related compounds.

2.2.4.6 Organic loading rate Organic loading rate (OLR) is a significant operation parameter that shows how much organic matter can be fed into the bioreactor, per volume and time unit, accordingly with the equation [3]:

BR = m * c / VR

Where: BR - organic load [kg/d*m³];

m - mass of substrate fed per time unit [kg/d];

c - concentration of organic matter [%];

VR - digester volume [m³];

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The possible hazard of a fast growth in the OLR is that the hydrolysis and acidogenic bacteria would generate intermediary products rapidly. Since the multiplication time of methanogenic bacteria is slower, they would not be capable to utilize the fatty acids at the same speed. The accumulation of fatty acids will cause a pH fall and hinder the activity of the methanogenic bacteria, inducing a system failure. Feeding the system above its OLR tolerance level causes low biogas output due to accumulation of inhibiting substances such as fatty acids in the bioreactor slurry [3].

2.2.4.7 Hydraulic retention time (HRT) A significant parameter of the bioreactor dimensional characteristic is the amount of substrate retention time. Hydraulic retention time is the average time interval where the certain substrate is held in the digester. HRT is correlated with the digester volume (VR) and the substrate amount fed per unit time, in accordance with the following equation:

HRT = VR / V

Where: HRT - hydraulic retention time [days]

VR - digester volume [m³]

V - volume of substrate fed per time unit [m³/d]

According to the equation above, an increase in the organic loading rate will decrease the HRT. Hydraulic retention time should be long enough to ensure that the amount of bacteria, which is removed with the digestate, is not larger than renewed bacteria (for example, anaerobic bacterial reproduction rate is 10 or more days). Short HRT provides a good substrate flow rate, but lowers gas volume output, so it is significant to adjust the HRT to the specific decomposition rate of the utilized substrates. Therefore, a target for duration of HRT is determined. Knowing the targeted retention time, the daily input of raw materials and the decomposition rate of the substrate, it is possible to calculate the volume of the bioreactor [3].

Simple municipal waste may cross through high-rate digesters, with HRT of only hours. However, multiplex waste, for example animal manure, must be digested at a HRT of 10 days or more because of the large proportion of recalcitrant organic substance present in cattle manure [63]. In a bioreactor with permanent mixing, the substance in the digester has a relatively equable retention time. In this case, the minimum HRT is determined by the growth rate of the slowest growing, substantial bacteria of the anaerobic microorganism community. If the HRT is too short, the system will get into disorder due to the rinsing away of the slowest growing bacteria that are necessary for the anaerobic process [64].

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3. Evaluation of the biogas production potential in the rural areas of Mexico

3.1 Overview of the energy sector The energy sector of Mexico is highly dependent on fossil fuels. Statistics show that Mexico is one of the leading oil producers in the world. Their main economical partner in the fossil fuels trade is USA. The earnings that Mexican budget gets from the export of crude oil were around 16% in 2011. The amount of produced crude oil in Mexico shows a declining trend, which might be interpreted as a passing of the peak production in 2004.

Figure 3.1 Mexico’s oil production and consumption [7]

Results of the decline in the production of fossil fuels directly reflect on the budget and fiscal health of the whole financial sector of the country. Proven reserves of oil are 10,2 million barrels which makes it a reliable source of income for the Mexican government. Experts expect no dramatic changes in technology or policy. Oil accounts for around 86% of the share of energy production, followed by natural gas, lease condensate and refinery processing gain. Second main energy source is natural gas. Despite the fact that Mexico has considerable resources, 480-bilion m3, the country is actually a net importer of natural gas, because of the low level of sector development [7]. Consumption of natural gas shows a growing trend mostly because of the need of electricity sector and petrochemical plants. The main import partner is USA who runs pipeline delivery, followed by other countries as LNG import partners.

Due to the facts described above, the electricity market and production of the country, which is one of the world’s highest (20th place in the world), are based on the fossil fuels. Oil, natural gas and coal are the main fuels used for electricity generation, which shows growing trend in last 30 years (Figure 3.2). The share of the renewables in the country’s electricity mix is about 18%, and the rest is covered by power generated by nuclear plants.

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Figure 3.2 Mexico’s net electricity generation [7]

97% of households in Mexico have access to the electrical grid, while more than 3 million people mostly living in the rural areas still have no connection to electricity [65]. Taking into account this fact as well as the decreasing trend in the oil production which also results in the drop of revenues from the fossil fuel sales, renewable energy is becoming a more and more attractive alternative. On the one hand it will solve the problem with energy delivery to the remote areas, and on the other hand it will allow the country to minimize dependence on non-renewable energy sources.

The current situation with renewables in Mexico is looking promising. It is possible to see increases in the amount of different development programs, sustainable projects and new initiatives. However, the market share is still very low, around 7% of the energy supply. There are several hydro, geothermal and solar power plants, but firewood contribution is still very high, around 30%, which points to the lack of technological development in some areas of the country.

3.2 Overview of the agricultural sector in Mexico Mexico has very long traditions of agriculture and animal breeding. Local farmers are facing a lot of problems due to high amounts of wastes produced, lack of proper sewage systems and high prices for energy sources in the areas located at a distance from the central systems, such as hot water and electricity [66]. (UNCTAD, 2014) This makes development of this economic sector, which accounts for around 6% of GDP, unsustainable in a country that is highly dependent on farming.

Agricultural activities themselves accounts for around 70% of the agricultural sector. Mexico is one of the leading suppliers for such agricultural products as sugar, vegetables, fruits, coffee etc. In this sector works 23% of the economically active population. But at the same time, the agricultural sector is still one of the mostly undeveloped sectors in the country’s economy. It is characterized by very low efficiency and profitability of production, lack of qualitative management and low product security and yields of crops. Prices for the agricultural products are tending to go down but cost of the production itself – fuel, machines and fertilizers, is growing. Salaries in the sector are below the minimal cost of living. For the last 10 years, the development of the sector is going down, which can be seen also in the GDP change.

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Figure 3.3 Agricultural map of Mexico [67]

The most important crop is corn. Mexico is one of the world’s leading producers of corn and highest consumers per capita – around 200 kg per year. Mexico is in the world top 5 of bean production and one of the leaders in soybeans. But in the last years, there is a new trend in the sector – the country is becoming more dependent on the import of agricultural products. The balance of the import and export in the last twenty years has shown a positive results only a few times. Basically, Mexico is becoming an importer from the world’s largest exporters. Main exporters of grain and oilseeds are USA and Canada. Mexican export is basically to countries of Central America.

Livestock related activities take 23% of the agricultural sector and is performed on 1 million km2. According to the information from SAGARPA (Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca, y Alimentación) there are around 2 million head of dairy bovines, 15 million head of pork, 31 million cows and beefs, 9 million goats, 7.5 million sheep, 300 million broilers in Mexico [68]. As it is shown on the agricultural map, livestock activities take place all over the country; however there is a difference in technological, ecological and production contexts. For example, beef production has the biggest market share in comparison with others. This agricultural market segment employs around 50% of the national territory and it is growing. However, the highest growth rate has been in the broilers business which has had a 6,9% increase in the last years. Another important fact is accelerated growth of the whole sector, which was demonstrated in the last two decades [66]. Level of technological development in the livestock sector is high enough to meet the need of almost 99% of the national market, as well as provide some agricultural products to the international markets. However, around 3 million producers still do not have access to technological developments, have very poor economic conditions and lack the possibility to sell production.

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3.3 Market supporting elements Diversity of the agricultural sector in Mexico is very high. It’s varying from small farms to huge commercially based productions and all of them have different economical and legal needs. Different policies and support schemes are aimed to face these challenges, economically protect local market players from the international agricultural producers, and minimize the consequences of the drawbacks in the agricultural economy. Study shows that the Mexican government is ready to invest in this sector’s R&D activities to support farmers making agriculture more profitable, reliable and sustainable.

3.3.1 SAGARPA SAGARPA - The Secretariat of Agriculture, Livestock, Rural Development, Fishing, and Food is the governmental organization which is aimed to support policies directed to overall sector development, growing production, financial protection for farmers, and promoting collaboration between government and producers. The budget of SAGARPA in recent years was 5.5 billion US dollars and is split between several agricultural support programs [68].

Figure 3.4 Structure of the SAGARPA budget [68]

The ''Support for Agricultural Income'' program accounts for about 24 percent of SAGARPA's budget. One of the most important subprograms is PROCAMPO (Programa de Apoyos Directos para el Campo) which was launched in 1994. Originally PROCAMPO was designed to provide economical support to the Mexican producers with the implementation of the North American Free Trade agreement. At the moment, each farmer cultivating crops on eligible land might receive support financing which is paid on the hectare basis.

Support for Agricultural

Income; 24%

Risk Prevention and Management; 23%

Support for Investment in

Equipment and Infrastructure; 20%

Sustainability of Natural Resources;

11%

Capacity Building, Techological

Innovation, and Rural Extension; 7%

Other; 15%

SAGARPA Budget structure

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The second support program, which accounts for 23% of budget, is ''Program of Risk Prevention and Management''. One of the subprograms guarantees to participating producers of selected types of crops that the level of their income will not fall below a certain reference level, determined by the government.

Next program is ''Support for Investment in Equipment and Infrastructure''. This program accounts for around 20% of the budget and includes a broad range of activities aimed to stimulate the growth of the agricultural sector. For example, support of technical improvements and agricultural mechanization, switching to more productive activities, food safety promotions, etc.

11% of the budget is accounted for by the program ''Sustainability of Natural Resources''. The program is aimed to minimize environmental impact from the agricultural sector and encourage farmers to use sustainable technologies and practices. A good example of it might be biogas production from the agricultural waste.

The last program that should be mentioned on this list is '' Capacity Building, Technological Innovation and Rural Extension''. Stimulation of activities in the area of technical assistance and increasing capacity are the aims of the program. A good example to be mentioned is the provision of technical advice and delivery of improved seeds and equipment to bean and corn producers [68].

3.3.2 Agricultural finance There are several governmental financial institutions in the area of agricultural finance. The main purpose of these is to increase the activity of commercial banking in the agricultural sector.

FIRA (Funds Instituted in Relation with Agriculture-Fideicomisos Instituidos en Relación con la Agricultura) was established in 1954 by the Mexican government and is run by Central bank of Mexico. The primary mission of this institution is offering financial and technical consultancy, guarantees and credits for the agricultural sector.

Another institution to be mentioned is Financiera rural, which replaced Banco Nacional de Crédito Rural (BANRURAL) which closed in 2003. This organization is focused on producers who do not have any access to other sources of financing by offering loans to farmers, promoting increase of capacity and level of development.

3.3.3 Renewable energy support schemes In recent years, the Mexican government has started development of various initiatives and support schemes for renewable energy sources. Huge breaks for the renewable energy sector were announced by the “Law for the Use of Renewable Energy and Financing of Energy Transition” known by its Spanish acronym LAERFTE, which became a very important step towards sustainable energy [69]. This law became a foundation for the development of the “National Strategy for Energy Transition and Sustainable Energy Use” and a “Special Program for Renewable Energy”. Regulations of renewable energy production, as well as utilization of clean energy generation technologies, are the main objectives of the LAERFTE which will be controlled by the Electricity Regulatory Commission and Ministry of Energetics of Mexico. These steps are aimed to support the expansion of renewables, decrease dependency on fossil fuels, develop financial instruments for the support of renewables and move the country towards sustainable future.

After the announcement of LAERFTE, there were developed a number of financial mechanism aimed to support renewable energy projects. The table below (Table 3.1) presents the information about

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different federal public funds, state funds, international funds as well as specific funds available for financial support for the projects that are designed to produce energy from sun, wind, water, biomass as well as geothermal resources.

Table 3.1 Available support schemes in Mexico

Institution Name of the

program / Mechanism

Type of mechanism

Eligible project phase

Eliggible geographic

area Website

BANOBRAS Project structuring Credit Construction Nation-wide www.banobras.gob.mx

NAFIN Sustainable

project support program

Credit Construction Nation-wide www.nafin.com

IDB

Clean technology

Fund/Climate Investment

Fund

Credit Studies Construction Nation-wide www.climateinvestment-

funds.org

World Bank International

Finance Corporation

Credit Construction Nation-wide www.ifc.org

BANOBRAS National

Infrastructure Fund

Credit Studies Construction Nation-wide www.fonadin.gob.mx

North American

Development Bank

Border Environment Infrastructure

Fund

Credit Studies Construction

US-Mexico border www.nadb.org

BANOBRAS Financial guarantee Guarantee Construction Nation-wide www.banobras.gob.mx

NAFIN Financial guarantee Guarantee Construction Nation-wide www.nafin.com

FONADIN Financial guarantee Guarantee Construction

Operation Nation-wide www.fonadin.gob.mx

BANCOMEXT FOMECAR Credit Construction Operation Nation-wide www.bancomext.gob.mx

It is important to mention that the Clean Development Mechanism of the Kyoto Protocol can also be used as a source of financial funds. Due to the fact that Mexico is a ”Non-Annex 1“ country, financial revenue from the trade of GHG reduction can be used as an additional support if the project is registered as a CDM project.

3.4 Willingness to change Recently, the Mexican government announced that climate change, safety and quality of the produced agricultural products as well as a decrease of the dependence on fossil fuels are among of the most serious challenges that Mexico is going to face in the following years [70]. (Fresh fruit portal, 2013) Problems connected with climate change are the matter of concern for the farmers producing

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vegetables and fruits. Huge amounts of products and land areas used for cultivation were damaged in the winters by severe frost and heavy rains in the summer periods. Climate change is becoming a real threat for the sector and a problem to think of for the government and farmers. Another challenge is safety and quality of the exported products. Very often in the media, information appears about the infection and contamination of the products from Mexico. GHG emissions, climate change and a decrease of the produced oil, which results in a loss of the revenue from the sales, made the government think about independence from fossil fuels. Shifting to renewable energy sources are aimed to solve all of these problems, as well as create new workplaces, deliver electricity to 3 million households in the rural areas of the country and finally create a greener image of the country on the global arena.

All these problems are appearing more often in the government’s agenda. Willingness to change is seen by the steps, initiatives and strategic development plans that government is applying. For example, renewable energy law “LAERFTE”, which was discussed before, should be seen as a step towards sustainability because it is aimed to promote the usage of clean technologies, renewable energy as well as give financial support to the energy producers. The prevention initiative is aimed to reduce the risk of contamination of agricultural goods in the process of production, processing and transportation. This initiative controls 16 elements of the production chain including the registration of manufacturers, their business history, and their use of water, hygiene practices, traceability, and more.

The situation described above is showing a good potential for biogas production from the agricultural wastes in Mexico. This renewable energy source will address most of the challenges described before as well as fit with the development strategies announced by the government. The usage of biogas will provide clean energy to remote rural areas, will address the problem of agricultural waste processing, improve the financial situation of the farmers as well as provide huge environmental benefits. Finally, this technology will give the opportunity to generate green energy which might be sold in form of electricity and hot water to the village households or if connection to the central grid possible even to the energy suppliers. Biogas production from agricultural waste might become a drive for sustainable development of the energy and agricultural sectors.

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4. Case study – Biogas production on the LA MONTAÑA dairy farm.

4.1 Introduction The idea of biogas production from the agricultural waste in rural areas of Mexico is aimed to provide a cheap and reliable energy source for locations where mostly fossil fuels are used for energy generation or in the remote rural locations that lack the connection to the local electricity grid. One of the farms that meet these requirements is “LA MONTAÑA” dairy farm located in Tizimín, on the Yucatán Península region in the south of Mexico.

Figure 4.1 LA MONTAÑA dairy farm

This dairy farm with 82 cows is located in the lowest milk production area in the country. Yucatán Península, located in the southeastern edge of Mexico, had very strong cattle ranching traditions, but since the middle of the 20th century has reoriented mainly towards tourism causing decrease of the farming production. It is also important to mention that this area lies within the Atlantic Hurricane Belt. Due to the climate change the hurricane frequency and strength has grown dramatically in the recent decades. In an area with a generally flat terrain the consequences of tropical storms can be catastrophic. As a result the farm is facing huge challenges with floods and damaged feeding crops, interrupted milking process and frequent electricity supply shortages. That is why this particular case might be very beneficial for the development of a biogas production project, which will help to solve a variety of problems ranging from local power shortages and energy security as well as addressing the major concern of global climate change.

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According to the information gathered about this farm, the herd size is 82 cows but most of them are very young which will lead to herd growth in the coming years. In this case it is optimal to design biogas production with account to the future increase of the herd size to 200 cows, which is basically giving the restrictions to the daily raw material capacity to around 10 tons of cattle manure per day.

Market research showed a comparatively high availability of technical solutions for the implementation of biogas production from agricultural waste. However, investment costs are also important aspect for this type of project, due to the need to attract investors for the raising of capital. Analysis of the available commercial companies in that field gave a good picture of the market situation and lead to establishing a contact with one Hong-Kong based equipment producer [77], which showed good results both in the technological and economy analysis. Technological project description, design and economy assessment in this study were carried out in cooperation with this company.

4.2 Biogas production unit Literature survey and theoretical research showed that biogas output as well as amount of produced energy is highly dependent on multiple factors, such as: unit construction, type of raw materials used, operation and control strategies. Biogas production from the cow slurry is a multi-step and a multi-technology process and for reaching the optimal output attention needs to be paid to each stage of the process flow already during the project design stage.

Figure 4.2 Designed biogas production plant combined with small-scale CHP

Taking into account the high availability of raw material on a constant basis, the continuous type of biogas production plant is chosen for the project. A general layout is shown in Fig. 4.2. Alternatively, a batch production plant to be filled and emptied after a certain calculated retention time can also be used, but requires a relatively intensive labor input and cannot guarantee stable gas output. Biogas plants operating in the continuous mode are more suitable for typical farms with stable raw material availability and can suit well into the dairy farm routine. In addition, gas output is more stable which gives advantages to the energy utilization and power production processes.

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4.2.1 Pre-separation of slurry To increase the energy density of the cattle manure pre-separation process is going to be implemented in the project. As it was found out before, dry matter content in the beef slurry can vary between 5 and 12 percent. After the separation process the moisture content can be decreased to around 70%.

Figure 4.3 Slurry separator [77]

According to the product specification the separator is equipped with oscillation device, which uses vibration in the separation process. That leads to rise of efficiency of the separation process as well as increase biodegradability of the biomass. Constructional features of the machine also allow fully automatic mode of operation, which is needed for the continuous type of plant described above. A slurry separator suitable for implementation in the hereby proposed project is shown in Fig. 4.3.

4.2.2 Biomass pre-treatment Chemical pretreatment with help of NaOH alkaline solution of the slurry is chosen for the project due to its simplicity and proven results. Literature survey showed that the alkaline method is the most widely used chemical pre-treatment method, which is able to increase the efficiency of solubilisation of COD (chemical oxygen demand) and as a result increasing the biogas extraction. Alkaline pre-treatment of substrates in anaerobic digestion is performed by adding alkaline solution on the substance at 25°C for 24 hours reaction time. For the pre-treatment process a special mixing pit is planned to be implemented directly after the separation process.

4.2.3 Biogas digester The most common materials for the digester construction are steel and concrete. For this particular project coated steel construction is chosen due to its long lifetime, chemical resistance and simple installation.

Hydraulic heating system will be inbuilt in the walls of the digester to ensure the thermophilic digestion mode, which requires around 50°C temperature. Thermophilic process regime is chosen due to its productivity advantages in comparison to other temperature regimes and due to the geographical location of the plant in a hot climate zone. Thermal insulation on the outer side of the reactor is needed to minimize heat loss to surroundings.

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Figure 4.4 Bioreactor [77]

The gas holder is planned to be placed on the top of the reactor, providing short-term storage of the produced gas. Flexible materials are usually used for the construction of this element. External protection film as well as impermeable membrane are the main components of the gas holder, however additional equipment such as security valves and pressure control need to be installed in the system.

To ensure the proper mixing intensity flat blade turbine mixers will be installed inside the digester. The main purpose of the mechanical mixer in the project is to ensure an intimate contact between the bacterial population in the digester and incoming fresh substrate.

Figure 4.5 Mechanical mixer [77]

Due to the fact that the system is designed to operate in a continuous mode, the mixer is vital for the stable and optimal process flow.

4.2.4 Biogas pre-processing High moisture content, hydrogen sulphide and ammonium are the impurities that can seriously damage the internals of the power generation unit. To avoid the increase of the maintenance costs as well as to

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keep the engine availability high, quality of the biogas has to be continuously controlled and the concentration of impurities measured.

To understand the relevance of quality control, it is important to find out what consequences can occur during the operation because of chemical and physical features of the biogas:

• Moisture content – humidity of the biogas is an important parameter because of condensation, which occurs as a result of temperature change. Generally after the process, anaerobic digestion biogas’s relative humidity is around 100%. As it can be seen on the psychometric chart, the biogas moisture content is in direct relation with gas temperature. Table 2.2 shows different types of temperature groups used in the production process and it should be noticed that produced biogas will have different water content. The reason for moisture content monitoring is the highly corrosive condensate due to chemical features of the substance which can cause equipment breakdown [71] (Steven Scott).

Figure 4.6 Biogas psychometric chart [72]

• Ammonium NH3 – taking into account the fact that ammonia is soluble in water, in the case of condensation it will create a corrosive solution that may destroy mechanical parts of the equipment. Another problem is nitrogen oxides (NOx) emissions that are increased by NH3. Concentration of ammonium in the biogas is highly dependent on the type of substance that is used for biogas production as well as chosen technology [73].

• Hydrogen sulphide H2S – is a toxic and corrosive gas usually present in biogas because of sulphur content in the raw material. Chemical reaction with water will result in the production of the extremely corrosive acid that can easily damage engine components. Acidification of the engine lube oil is another problem caused by the hydrogen sulphide. From the environmental point of view, H2S is also a problem due to the increase of SO2 emissions in the atmosphere [73].

All elements mentioned above may cause serious damage to the power generation system in the case of uncontrolled use of biogas. Usually biogas produced with anaerobic digestion technology does not meet the requirements of the engine producer, who is setting a claim for the fuel quality for the normal operation of the unit. In this case, pre-processing of the biogas is required.

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Figure 4.7 Biogas conditioning system [77]

The following operations will be carried out to pre-process the gas before utilization:

• Elimination of the ammonium and hydrogen sulphide; • Reduction of the moisture; • Filtration of the dust and oil drops; • Relative humidity, temperature and pressure are the parameters that also should be controlled

and adjusted during the pre-treatment process;

In the case of well-planned biogas pre-processing, continuous monitoring and analysis of the parameters, the availability, efficiency and the lifetime of the power producing equipment will be significantly increased. This will lead to higher financial revenues as well as stable and maintenance-free power plant operation.

4.2.5 Automatics For the economic operation of this kind of system, a lot of parameters, such as temperature, pressure, pH values etc. should be continuously controlled and analyzed.

Figure 4.8 Biogas plant control unit [77]

The automation system should be able to control production unit automatically. Nowadays it is very common to connect such systems to the Internet for the possibility of remote control and monitoring of

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the process parameters. Special software designed for the control of biological process with a user-friendly interface is needed for farm-workers to work with this equipment.

4.3 Biogas utilization with CHP The most energy effective way to utilize biogas produced in a rural area is a local small-scale combined heat and power unit (CHP), which is able to produce electricity as well as heat simultaneously with high efficiency in the so called cogeneration process. Utilizing this kind of technology is responsible from the environmental point of view and it also has very attractive financial benefits. However, there are specific requirements for biogas quality to be capable for utilization in CHP unit, which were discussed in the previous chapters.

As it was mentioned before, CHP units are capable of generating multiple useful forms of energy in the sequential or simultaneous generation process. In comparison to the conventional power generation unit, CHP technology utilizes waste heat, which can be used for heating of buildings and water as well as being applied to the biogas production process.

Figure 4.9 CHP unit components [74]

CHP is an integrated system that consists of different components: prime mover, power generator, as well as heat recovery. The whole process is driven by the primary mover, which actually determines the type of CHP system. The most common primary mover types that are used in the mini-CHP units are fuel cells, closed cycle steam engines, stirling engines, micro turbines and internal combustion engines [75]. A variety of possible prime movers gives flexibility in the type of fuel utilized for the energy production - oil, natural gas, coal as well as alternative fuels are suitable for utilization to produce mechanical energy which is converted to electricity by the power generator.

From the thermodynamic point of view, the cogeneration process is a very efficient type of fuel utilization because the excess heat is also put into operation, while in comparison to conventional electricity production it is directly dumped to the environment. Possibility to capture by-product heat from the electricity generation process is the biggest advantage of the CHP technology which can reach total energy utilization efficiencies of up to 95% [76].

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4.4 Project economics From the environmental point of view, local utilization of biogas for power generation is an extremely competitive alternative in comparison to fossil fuels such as oil and gas. However, the financial aspect of this technology needs to be analysed to show that the project can also be profitable from the economical point of view. Different aspects of the biogas production, such as equipment costs, lifetime of the system and components, maintenance and operation (O&M) costs, as well as interest or discount rates need to be taken into account. As it was mentioned before, system design and economy analysis is done in cooperation with a Hong Kong based equipment producer [77], which has a wide experience with project implementation in the Pacific region.

Table 4.1 Sample price-list for biogas production unit [77]

There are different types of economy analysis methods and models but all of them have to be based on the reliable assumptions of the capital (or total investment) costs, which generally describes the cost of the biogas production unit itself and its installation, as well as O&M costs, which may vary based on the different factors. Calculation of the financial performance of the project, as well as a proper comparison between the prices for different energy sources such as biogas and fossil fuels at the selected location, are the main purposes of the economy analysis.

To get the overall understanding of the biogas production economics, different types of economic analysis models will be used:

Payback Time Analysis; Cost of energy method;

These methods are commonly applied for the determination of the economic benefits of energy production technologies. They are relatively simple for understanding and calculation, but very valuable from the economic point of view. All costs further below are in USD as per year 2013.

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4.4.1 Payback time analysis A calculation of the payback time gives an indication of the important economic parameter – time to redeem an initial investment. A simplified model of the calculation consists of such parameters as total capital cost of the biogas production unit and average annual refund from the energy produced by biogas utilization. The payback period is expressed by the simplified equation:

𝑃𝑇 =𝐶𝑐𝐴𝐴𝑅

Where: PT – payback time;

Cc – total capital cost [$];

AAR – average annual refund [$];

The average annual refund for the biogas production unit can be expressed as:

𝐴𝐴𝑅 = 𝐸𝑎𝑃𝑒

Where: Ea – annual energy production (kWh/year];

Pe – price obtained for electricity [$/kWh];

The simplified calculation is carried out for the sample project, which has a capacity of 10 tons of cattle manure daily availability (200 cows’ farm). Average investment cost for the biogas production plant can be seen in Table 4.1 and the price for a suitable CHP unit is 1400 $/kW [78]. Typical electric efficiency for small CHP units utilizing biogas is around 35% [78]. Due to the fact that in Mexico there is no central heating network, there is no possibility to sell heat, so the net income calculation will be carried out for electricity selling only. Knowing that the electricity price in Mexico is based on a tariff plan and on consumption, the electricity price used in this calculation is an average price of the cheapest and most expensive tariffs for the domestic or household use – 1.7065 pesos/kWh which is equal to 0.13$/kWh [79]. Specific electricity yield for the biogas produced from liquid cattle manure can be approximated to 1.98 kWh/m3 [78]. Another assumption that should be done is the biogas production plant availability, which in this case is assumed at 350 days annually, whereas during 15 days a year it is closed for maintenance. Taking into account all the data, annual energy production Ea can be calculated as:

Ea = 650 x 350 x 1.98 = 450 MWh/year

Assuming that around 10% of electricity is used for the biogas production, the amount that is available to sell is estimated to be 400 MWh/year. In this case, the average annual refund is:

AAR = 400000 x 0.13 = 52 000 $

To calculate the payback time, total capital cost Cc needs to be considered:

Cc = 219 000 $ [biogas production plant] + 79 000 $ [56kW CHP] = 298 000 $

Payback time in this case is:

PT = 298 000 / 52 000 = 5.7 years

As was mentioned above, this is only a simplified payback calculation that does not take into account a lot of factors which can influence payback time significantly. Factors such as O&M costs, electricity price

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variations, components lifetime etc can have a significant effect on the calculated parameter, which is why the obtained number might be used only as an indicator for the preliminary decision of the financial aspects of the mentioned technology.

4.4.2 Cost of energy method Another mechanism for better understanding of the economics of biogas production is a cost of energy analysis. This method gives an understanding of the cost to produce energy and gives a possibility to compare the price in $/kWh with other alternative energy sources. Cost of energy can be calculated using the following equation:

𝐶𝑂𝐸 =𝐶𝑇𝐸𝑎

Where: COE – cost of energy [$/kWh];

CT – total cost [$];

Ea – annual energy production (kWh/year];

For the total cost (CT) calculation it is important to know fixed-charge rate (FCR) which presents the expenses that occur on a regular basis and is commonly used for creation of a predictable budget as well as cash flow estimations. Another important parameter is O&M annual cost, which can significantly influence the price of the produced energy. Knowing both of these factors, the total annual cost (CT) can be estimated by the following equation:

𝐶𝑇 = 𝐶𝐶 × 𝐹𝐶𝑅 + 𝐶𝑂&𝑀

Where: FCR – annual fixed-charge rate [%];

Cc – total capital cost [$];

CO&M – average annual operation and maintenance cost [$];

Annual fixed-charge rate can vary a lot because of different financial aspects, such as insurance, taxes, depreciation etc, however for the power production unit, FCR can be approximated to 15%. (Shaalan) An average annual operation and maintenance cost for the biogas production unit and CHP are approximated at 18.5 $/1000m3 gas and 21 $/MWh electricity, respectively [78]. Taking into account all the assumptions made above, total annual cost (CT) is:

CT = 219000x0.15 + ((227.5x18.5)+(450x21)) = 47 000 [$]

Using the annual energy production Ea from the previous section, the cost of energy produced by the proposed system can be calculated as:

COE = 47000/450000 = 0.11 [$/kWh]

This result shows that electricity production from the biogas utilizing CHP technology can be competitive with other energy sources. However, this analysis is based on assumptions, which are not taking into account inflation, interest rates and the time value of money.

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5. Conclusion This study is based on information gathered from Mexican governmental organizations and agencies as well as biogas production technologies and prices available on the market nowadays. The main purpose of this research was to study the possibilities of energy delivery to the rural areas of Mexico that still lack the connection to the national electricity grid as well as to farmers that might be interested to investment in renewable energy production using governmental support schemes. The results of the study are aimed to be the base for future work towards development of small-scale biogas production projects all over the country.

The results show that biogas energy is an attractive and beneficial option for decreasing the dependence on fossil fuels, increasing energy availability in rural areas and resulting in a big step towards a more sustainable future for Mexico. Gathered information shows high availability of feedstock for biogas production because of the significance of livestock production in the economy all over the country’s territory. Appropriate ambient air temperature leads to conditions where biogas can be produced throughout the year with relatively low addition of heating energy to the process. New workplace creation will lead to a decrease in the unemployment rate, financial stability and an increase of profitability in the agricultural sector. The huge number of inhabitants - around 3.6 million people, living in rural areas that still lack access to electricity, can be significantly decreased by implementation of the described biogas production and utilization technology.

Another important aspect that was studied in this paper is the willingness to change. In recent years, the Mexican government has developed a significant amount of support schemes and policies for the rural agricultural and renewable energy sectors. All of these initiatives are aimed to develop and financially support projects like the one described in this paper. Due to high availability and low prices of fossil fuels in Mexico, it is hard to compete with the big energy market players; however, the mentioned governmental support should be seen as a backing for the local renewable energy producers, mostly situated far from the densely populated centres. Available loans and governmental guarantees are a very good base for the creation of the conductive circumstances for the investors, and as a result, stable financial flows of the project.

The attempted conceptual design of a biogas production unit for the selected dairy farm in the south of Mexico and the economic analysis based on the mentioned project parameters showed that electricity production utilizing biogas with CHP technology is financially competitive with conventional energy available on the Mexican market. The average price for electricity in Mexico is 1.7065 pesos/kWh which is equal to 0.13 $/kWh. However, the cost of electricity from the CHP plant is calculated at 0.11 $/kWh, which is a very good indication for the possible financial benefits. The analysis also showed a relatively low payback time for the sample project of 5.7 years. Taking into account both these numbers as well as the financial guarantees provided by the government, the financial feasibility of the developed project can be ranked as very high.

All these findings lead to the conclusion that small-scale biogas production plants in association with CHP units for rural Mexico are a very appropriate option for solving the variety of problems that are faced all over the country. Cheap and reliable biogas technology will provide a lot of benefits to the local population, country’s economy, as well as have a positive effect on the global climate change problem.

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5.1 Future work During the study, a sample biogas production and utilization project was developed for the dairy farm. Control and operation of the biogas production unit were studied in depth. The theoretical base given in the paper is enough to bring this type of project to life and successfully operate it in an efficient way. The gathered information gives an overview of the economic and political instruments which can be used for biogas project financing all over the country. The practical applicability of each of the available governmental support schemes needs to be further examined in connection to specific project applications at specific locations.

The present study attempted a simplified economy analysis for the payback time and the cost of electricity produced by the proposed case scenario. However, these economical models are based on assumptions and approximations that do not take into account inflation, interest rates and the time value of money. Moreover, the operation and maintenance costs as well as other variations in costs and input or output parameters have not been properly assessed. Future work should include a more thorough economical assessment that will consider all the mentioned economic parameters including a proper LCCA estimation, as well as equipment prices available on the local market in Mexico.

In designing and operating biogas production plants, the following points should be taken into account:

Designed system should be stable and functional in the long-term; Proven technologies should be chosen for all the system components; Control and operation strategies should be very well planned; Safety standards should be considered; Education and training for the staff should be provided; Maintenance of the plant should be done by authorized personal;

Design and construction of the biogas production plant should take into account specific needs in each particular case. Capacity of the biogas production should be adjusted to the amount of raw material available; however this will not be a problem due to a variety of different technologies available on the market. In some cases, combination of different types of renewable energy sources can be a solution. Design of a hybrid energy production system should be also considered for future research.

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